Bending presses are machine tools of a type that is itself well known. As shown in accompanying FIG. 1A, the machine tool comprises a lower table 12 and an upper table 14 that is movable relative to the lower table 12. Usually, the lower table 12 is stationary and the upper table 14 is suitable for being moved towards the lower table 12 under drive from actuators V1 and V2 that act on the ends 14a and 14b of the upper table. Usually, the lower table 12 has its free edge 12a fitted with fastener means 16 for fastening bending matrices 18. In the same way, the edge 14c of the upper table 14 is fitted with fastener means 20 for fastening bending punches 22.
A metal sheet or lamination F is placed on the bending matrices 18 of the lower table 12. The sheet F may be of a length L that varies widely depending on circumstances. Under drive from the pistons of the actuators V1 and V2, the punches 22 mounted on the upper table move towards the sheet F placed on the matrices of the lower table. As soon as the punch comes into contact with the sheet, force begins to increase within the metal lamination or sheet F as the punch penetrates therein, initially in the elastic range and subsequently in the plastic range, thereby enabling the sheet to be bent permanently.
Because the force is applied to the upper table by the actuators V1 and V2 acting on the ends of the table, the linear load distributed between the two ends of the tables corresponds the upper table being deformed along a line in the form of a concave arc with deformation maximas close to the midplane of the table. This means that for bending purposes, at the end of bending, the central portions of the punches have penetrated into the sheet less than have the end portions. If bending were to be performed on a matrix that, itself, were to remain perfectly straight during bending, then a metal lamination or sheet F would be obtained having a bend angle that was wider in its central portion than at its ends. Such a result would naturally be unacceptable.
In reality, the matrices carried by the lower table, or more precisely by the free edge of the lower table, are in fact subjected to deformation during bending, which deformation is likewise concave with its maximum in the central portion. The result of these two deformations is that, in reality, the bending obtained in the sheet is very open in the middle portion of the press and very closed at its ends. In reality, the difference may reach an angle of several degrees, e.g. 93° at the midplane of the tables and 90° at its ends. The resulting sheet thus presents poor accuracy concerning the linearity of its bend, thus giving it a so-called “boat” shape.
In order to remedy that drawback, various solutions have been proposed for the purpose of controlling these deformations at the edges of the tables by using various means in order to obtain a bend that is substantially identical over the entire length of the bent metal lamination or sheet F.
Usually, these solutions involve providing slots, such as the slots 24 and 26 shown in FIG. 1A, that are formed in the lower table symmetrically about the midplane of the press. These slots then define a central zone 28 of the lower table that is slot-free and that presents a length l0 together with two slots 24 and 26, each of length a.
With slots 24 and 26 of conventional type, i.e. that leave between them a slot-free portion 28 of long length l0, substantially parallel deformations D1 and D2 are indeed obtained for the edges of the upper and lower tables 14 and 12, as shown in FIG. 1B. This ensures that proper bending is achieved. Nevertheless, this result is obtained only when the metal lamination or sheet for bending has a length that is substantially equal to the total length of the lower or upper tables. In contrast, with the known solutions and as shown in FIG. 1C, when the length of the sheet is shorter than the total length of the lower or upper table, both of the deformations D′1 and D′2 are concave. Japanese utility model 2 558 928 in the name of AMADA CORPORATION describes a solution in which both slots in the lower table are provided with respective movable members of positions that are adjustable within the slots. Those movable members are directly in contact with the bottom and top edges of the slots. Nevertheless, that solution enables satisfactory results to be obtained only for certain lengths of metal sheet relative to the total length of the press, but not for others. Furthermore, it does not take into consideration problems associated with the fact that the metal sheet may occupy a position that is asymmetrical relative to the midplane of the bending press, while nevertheless enabling a bend to be obtained that is identical along the entire length of the metal sheet.
In European patent EP 1 112 130, there is proposed a solution of fitting each slot of the lower table with a mover member connecting together the free top portion and the stationary bottom portion of the lower table so as to cause the top wall of the slot to approach the bottom wall thereof, assuming the bottom wall is stationary. This enables the curvature of the deformation of the free top portion of the lower table carrying the matrices to be modified in controlled manner, in the absence of any stress being applied to the tables. The drawback of that solution is that it requires a complex hydraulic control installation to be put into place.
Similar solutions are disclosed in documents JP 2001-71033, JP 2000-343125, and WO 01/43896.